STRATIGRAPHIC AND BIOTIC PUNCTUATION OF DIKELOCEPHALID TRILOBITE EVOLUTION IN THE UPPER MISSISSIPPI VALLEY CAMBRIAN

Nearshore settings have been invoked as the sites of evolutionary novelty both in the fossil record as well as some contemporary studies. The early Paleozoic deposits of the Upper Mississippi Valley nearshore basin offer an opportunity to consider how observed patterns of morphological change relate to sedimentary accumulation processes in the basin. The sedimentary record of this basin is condensed due to its nearshore setting, but nonetheless provides quite a comprehensive record of sedimentary accumulation of this time. The apparently abrupt appearance of derived dikelocephalid taxa in the area is of two kinds. The genus Osceolia exemplifies the appearance of a new morphotype associated with a break in the stratigraphic record – in this case a maximum flooding surface. Within its temporal span of likely less than 1.5 million years, its members reveal a complex pattern of ontogenetic and phenotypic variation in its representation across ~24 parasequences deposited in the early part of a falling stage systems tract. This variation includes novelties in pygidial spine form and anterior border structure, some of which are intra-collectional, and which accord with standard patterns of microevolutionary phenotypic variation. In marked contrast, the sister taxon of the locally abundant and long ranging Dikelocephalus minnesotensis, Walcottaspis vanhornei, makes an abrupt appearance and exit from the fossil record entirely within the falling stage systems tract, and likely endured for only a few hundred thousand years at most, and possibly less than one hundred thousand years. Although it also shows marked phenotypic variance within the species, its abrupt appearance and disappearance within the range of D. minnesotensis argues for a genuine punctuated mode of evolutionary appearance and disappearance that is not an artifact of sediment accumulation history.